1. Field of the Invention
This invention relates to a method for bonding steel plates with a polyamide-type adhesive and to a composition for use therein. More specifically, the invention relates to a method for bonding a steel plate wherein, prior to bonding, a composition comprising (1) a polyester resin obtained by reacting a polycarboxylic acid component with a polyhydric alcohol component and (2) a bisphenol A-type epoxy resin, is coated on the steel plate, and baked by heating.
2. Description of the Prior Art
According to the prior art techniques, many metallic containers such as cans for foodstuffs and general-purpose cans are made of steel plates with tin, the so-called tin plates, and the can body blanks are bonded by soldering. However, bonding by soldering has the defect that rust may occur in the cans due to hydrochloric acid, which is used to clean the soldered parts, remaining on the can, the aesthetic appearance of the cans may be impaired by the lack of printing at the soldered parts, or fumes of lead, a component of the solder used, may give rise to toxicity hazards.
In recent years, use of chromium-treated steel plates (TFS: tin-free steel-chromium type) which are less expensive and have superior printability have superseded the use of tin plates. Since this type of material cannot be soldered, methods have been developed to bond can bodies using an adhesive.
In many cases, polyamide-type adhesives are used to bond a can body using an adhesive. When polyamide-type adhesives are used, an adhesive primer is generally used. The primer actually used is a coating composition comprising a phenol formaldehyde/epoxy resin. However, when a primer of this type is used, the resin-coated surface turns to a yellow brown color upon baking, and the beauty of the surface is impaired.
Hence, when a composition of this type cannot be used as a varnish coat to be coated on an outside surface of can body because the coated film is colored, or high temperatures are required for curing the coated film by baking.